X-rays are often used for the inspection of enclosures such as cargo containers. The use of x-rays both transmitted through the inspected item and scattered by its contents are known in the art. Typically, the item is illuminated by a single source of x-rays, and transmitted and scattered radiation is detected by detectors or arrays of detectors disposed, respectively, in the direction of propagation of the illuminating beam or in other directions, to the front, back, or sides of the inspected item.
To obtain useful images of the x-rays transmitted through heavily loaded containers, such as truck trailers or sea shipping containers, etc., often 6 feet to 10 feet thick, it is necessary to use x-rays with energies well in excess of 1 MeV. Lower energy x-rays may be attenuated to the point that they can no longer be detected by detectors on the side of the container opposite to the side illuminated by the x-ray source. Typical commercial x-ray systems such as those employed for the purposes described above use linear accelerators to generate x-rays from electrons having energies from 5 MeV to 10 MeV; the average energy of the x-ray spectrum generated by a 10 MeV machine is approximately 3 MeV.
Images obtained from transmitted x-rays, however, are insensitive to thin, light material. Scattered radiation, in particular backscattered radiation may be used to obtain images of such thin objects not too far inside the containers and also to discriminate organic from non-organic objects. Various methods of identifying a backscatter signal with a position within the illuminated object employ scanned pencil beams of x-rays, are described, for example, in U.S. Pat. Nos. 4,809,312 and 4,825,454 which are hereby incorporated herein by reference.
At lower x-ray energies, i.e. &lt;.about.450 keV, it is practical to obtain a transmission image and a backscatter image simultaneously using the x-rays from a single x-ray generator. As the energy of the x-rays increases, however, the method of backscatter imaging becomes less and less practical for three reasons: First, the probability of scattering into the back directions (i.e. through about 180.degree.) drops rapidly as the energy of the x-rays increase. Thus MeV x-ray beams produce relatively weak backscatter signals. Second, the backscatter signal becomes independent of the scattering material for x-ray energies above a few hundred keV. Thus, backscattering from beams having energies in the MeV range, such as those discussed in connection with transmission imaging of large containers, provide insufficient material discrimination. Third, while backscatter imaging typically employs a "pencil" beam of x-rays to raster across the container, such that the cross sectional dimensions of the beam determine the spatial resolution of the backscatter image, it is technically difficult to form a scanning pencil beam of MeV x-rays.